This invention relates to an apparatus and method for discriminating between ferrous and non-ferrous metal targets.
Traditionally there are two types of metal detectors both of which illuminate the ground with a magnetic signal, namely:
Type 1. One type is used for gold recovery which supplies an audio signal to the user which is modulated by ground minerals and usually all conducting metals. The ground signals give a continuous relatively slow varying background signal. Gold gives a shorter transient signal. Hence, the operator has to try to notice subtle differences in the transients of the audio signal to assess whether a gold nugget is being interrogated. The interfering background minerals adversely affect the depth to which these gold nuggets may be located. Art in four patents namely U.S. Pat. Nos. 4,942,360, 4,894,618, U.S. patent application Ser. No. 07/468,471, filed Jan. 12, 1990 (abandoned) and U.S. patent application Ser. No. 07/501,106, filed Mar. 29, 1990 (abandoned) describe means of overcoming the above difficulties.
Type 2. The second type of detector is more common and is used to locate valued man made artefacts amongst non-valuable metals, eg. coins and jewellery amongst rusty iron and beverage can pull-tabs. This is possible because the different metals have different magnetic properties in the case of iron versus non-iron, and specific electrical conductivities in the cases of non-iron junk such as aluminium pull-tabs compared to coins. These detectors usually only produce an audio signal when a potentially valuable non-iron object type of signal is being detected. It should be noted that most patents in this field relate to this type of metal detector.
This invention relates to the second type of detector. In particular, it relates to the differentiation between iron and non-iron objects buried in iron oxide mineralised ground; the most common type of interfering mineral. When applying an alternating magnetic field to an environment, such as a volume of ground containing a target, the environment is modified in three different ways, these being:
1) by the generation of a magnetic field which depends only on the history of the applied magnetic field resulting from energy dissipated from the applied field, PA1 2) by an instantaneous and direct enhancing of the applied field known as ferro-magnetism or ferri-magnetism, PA1 3) by an instantaneous and direct opposing of the applied field known as dia-magnetism. PA1 a transmission means for providing a first alternating magnetic field in an area of ground, the first alternating magnetic field containing at least two frequency components these being a first frequency component and a second frequency component such that the second frequency component's frequency is higher than the said first frequency component's frequency; PA1 a receiving means for providing a resultant voltage dependent upon a second magnetic field resulting from the effects of the ground, and the effects of targets buried in the ground, upon the first alternating magnetic field; PA1 a sampling means for sampling at least three voltage measurements of the resultant voltage over three different time periods to provide three sampled values, the sampling being synchronised to the first alternating magnetic field; PA1 a processing means for both processing the three sampled values to provide three average ground balanced signals and providing at least two ratios from the averaged ground balanced signals; PA1 a comparison means for comparing the at least two ratios against a pre-defined response; and PA1 an informing means responsive to the comparison means and adapted to provide an informing signal indicative of a possible location of any target of selected nonferrous character in response to any comparison resulting from the comparison means. PA1 a electrical supply means for providing a voltage to a magnetic transmission means, the electrical supply means providing a first abrupt transition from a zero voltage to a substantially steady state voltage for a period of time after which the pulse generation means abruptly removes the substantially steady state voltage from the magnetic transmission means such that a resultant back electro motive force is produced such that the transmission means emits a first alternating magnetic field in an area of the ground; PA1 a receiving means for providing a resultant voltage dependent upon a second magnetic field resulting from the effects of the ground, and the effects of targets buried in the ground, upon the first alternating magnetic field; PA1 a sampling means for sampling at least three voltage measurements of the resultant voltage over three different time periods to provide three sampled values, the sampling being synchronised to the first alternating magnetic field; PA1 a processing means for both processing the three sampled values to provide three average ground balanced signals and providing at least two ratios from the averaged ground balanced signals; PA1 a comparison means for comparing the at least two ratios against a pre-defined response; and PA1 an informing means responsive to the comparison means and adapted to provide an informing signal indicative of a possible location of any target of selected nonferrous character in response to any comparison resulting from the comparison means. PA1 an electrical supply means for providing a voltage sequence of at least two different pulse periods to a magnetic transmission means, the pulse periods each consisting of a first abrupt transition from a zero voltage to a substantially steady state voltage for a the duration of the said period, after which the generation means abruptly removes the substantially steady state voltage from the magnetic transmission means which results in a resultant back electro motive force such that the transmission means emits a first alternating magnetic field in an area of the ground; PA1 a receiving means for providing a resultant voltage dependent upon a second magnetic field resulting from the effects of the ground, and the effects of targets buried in the ground, upon the first alternating magnetic field; PA1 a sampling means for sampling at least three voltage measurements of the resultant voltage over three different time periods to provide three sampled values, the sampling being synchronised to the first alternating magnetic field; PA1 a processing means for both processing the three sampled values to provide three average ground balanced signals and providing at least two ratios from the averaged ground balanced signals; PA1 a comparison means for comparing the at least two ratios against a pre-defined response; and PA1 an informing means responsive to the comparison means and adapted to provide an informing signal indicative of a possible location of any target of selected nonferrous character in response to any comparison resulting from the comparison means. PA1 a transmission a first alternating magnetic field in an area of ground, the first alternating magnetic field containing at least two frequency components these being a first frequency component and a second frequency component such that the second frequency component's frequency is higher than the said first frequency component's frequency; PA1 providing a resultant voltage dependent upon a second magnetic field resulting from the effects of the ground, and the effects of targets buried in the ground, upon the first alternating magnetic field; PA1 sampling at least three voltage measurements of the resultant voltage over three different time periods to provide three sampled values, the sampling being synchronised to the first alternating magnetic field; and PA1 processing the three sampled values to provide three processed signals substantially independent of the effects of iron oxides in the ground, and providing at least two ratios from these three processed signals substantially independent of the effects of iron oxides; PA1 comparing the at least two ratios against a pre-defined response; and PA1 producing an informing signal from an informing means responsive to the said comparison which will be indicative of a possible location of any target of selected nonferrous character. PA1 providing a voltage to a magnetic transmission means to provide a first abrupt transition from a zero voltage to a substantially steady state voltage for a period of time; PA1 abruptly removing the substantially steady state voltage from the magnetic transmission means such that a resultant back electro motive force is produced such that the transmission means emits a first alternating magnetic field in an area of the ground; PA1 providing a resultant voltage dependent upon a second magnetic field resulting from the effects of the ground, and the effects of targets buried in the ground, upon the first alternating magnetic field; PA1 sampling at least three voltage measurements of the resultant voltage over three different time periods to provide three sampled values, the sampling being synchronised to the first alternating magnetic field; and PA1 processing the three sampled values to provide three processed signals substantially independent of the effects of iron oxides in the ground, and providing at least two ratios from these three processed signals substantially independent of the effects of iron oxides; PA1 comparing the at least two ratios against a pre-defined response; and PA1 producing an informing signal from an informing means responsive to the said comparison which will be indicative of a possible location of any target of selected nonferrous character. PA1 providing a voltage sequence of at least two different pulse periods to a magnetic transmission means, the pulse periods each consisting of a first abrupt transition from a zero voltage to a substantially steady state voltage for a the duration of the said period, after which the generation means abruptly removes the substantially steady state voltage from the magnetic transmission means which results in a resultant back electro motive force such that the transmission means emits a first alternating magnetic field in an area of the ground; PA1 providing a resultant voltage dependent upon a second magnetic field resulting from the effects of the ground, and the effects of targets buried in the ground, upon the first alternating magnetic field; PA1 sampling at least three voltage measurements of the resultant voltage over three different time periods to provide three sampled values, the sampling being synchronised to the first alternating magnetic field; PA1 processing the three sampled values to provide three processed signals substantially independent of the effects of iron oxides in the ground, and providing at least two ratios from these three processed signals substantially independent of the effects of iron oxides; PA1 comparing the at least two ratios against a pre-defined response; and PA1 producing an informing signal from an informing means responsive to the said comparison which will be indicative of a possible location of any target of selected nonferrous character.
There is no energy dissipated from the applied field in ferro-magnetism, ferri-magnetism or dia-magnetism.
When measuring the above environment modifications the component of the resulting signal which depends upon the history of the applied magnetic field is known as the loss component because energy is dissipated. When referring to applied sinusoidally varying magnetic fields this loss component is known as the resistive component.
When measuring the ferro-magnetism, ferri-magnetism or dia-magnetism components, the resulting signal is known as the purely magnetic component. When referring to applied sinusoidally varying magnetic fields this component is known as the reactive component. It should be noted that dia-magnetic component is opposite in sign to the ferro-magnetism or ferri-magnetism component. Hence, the dia-magnetic component can be considered as a "negative" purely magnetic component.
A limitation with traditional detectors is that they endeavour to measure the ferro-magnetic properties of iron junk amongst a random background of magnetic iron oxides in the ground, in particular, haematite and magnetite. For deeply buried targets, the background signals are often much stronger than those of the metal target. Hence, even though it is possible to determine the presence of the metal target for the well known reasons given below, it is very difficult to determine whether a deeply buried target is iron or non-iron because of the interfering ground signals.
The apparatus in the following patents all measure the reactive component to determine whether a target is ferrous (ferro-magnetic, eg., iron) or non-ferrous (non-ferromagnetic, eg., silver, aluminium): GB 1,350273, U.S. Pat. Nos. 4,677,384, 4,677,384, 4,128,803, 4,249,128, 4,486,713, 4,700,139, 4,325,027. Most also have means to determine the conductive nature of the target so that further assessment can be made about the likely type of non-ferrous target being interrogated. This is so that aluminium foil or pull-tabs may be discriminated against. Other patents such as GB 2,004,069, Pat. Nos. 4,628,265, and 4,942,669 describe forms of apparatus using ground balanced multiple frequencies to determine the conductive nature of non-ferrous targets more accurately without magnetic ground interference. Some of these cited patents, for example U.S. Pat. Nos. 4,677,384, 4,700,139 and 4,128,803 describe means of high-pass or band-pass filtering to reduce the effect of the relatively slow varying background ground signal while relatively enhancing the transient "point source" signal of metal targets, in order to reduce the effects of the ground reactive component contaminating the target signal reactive component, and hence substantially limiting discrimination accuracy. This filtering technique is used in almost all type 2 commercially available metal detectors. However, while this filtering technique is a significant improvement over no filtering, the discrimination depth in most grounds is still significantly reduced compared to the depth to which targets can be located without the ability to assess their ferrous/non-ferrous properties.
Patent specification U.S. Pat. No. 4,110,679 describes an apparatus that transmits a complex triangular current waveform. The apparatus transmits a high current pulse that produces a magnetic field strong enough to cause measurable hysteresis in iron targets. This powerful signal is followed by a period of non-transmission sufficiently long for eddy current in metal targets to become insignificantly small. Then a smaller or series of smaller pulses are transmitted in the opposite polar sense. During this transmission, the received signal is measured. As the iron targets partially "unsaturate" they produce a different signal relative to non-ferrous targets which of course have no magnetic hysteresis. The signal processing is arranged so that the processed received signal does not respond to the purely reactive component. However, this discrimination action only works satisfactorily in some ground areas and then only on targets that are not deeply buried it is the iron targets which saturate and not the magnetic ground. This is because the magnetic field decreases rapidly with increasing distance from the transmit coil.
All of the second type of metal detectors described above are required to distinguish between ferrous and non-ferrous targets. Typically, more than 95% of all metal buried in the ground is ferrous such as rusty nails, screws, "tin" cans, tacks and pieces of fencing wire. All commercial detectors achieve this discrimination by comparing both the measured loss component and the purely magnetic component in which their relative signs are fundamental to this discrimination. However, this comparison has limitations when assessing the nature of the target. These limitations occur owing to two reasons. Firstly, in large ferrous targets the eddy current time constant may be long and the dia-magnetic component of the eddy current may dominate the ferro-magnetic component and mask its presence. That is the dia-magnetic component may exceed the ferromagnetic component. Hence such long time constant eddy current ferrous targets are often incorrectly determined to be non-ferrous targets by all existing metal detectors. Secondly, for relatively deeply buried targets it is possible to measure the loss component in a substantially ground balanced channel (described below), but of the target signal magnetic component can be insignificant compared to that of the ground signal. Thus, it can be impossible to determine whether a deeply buried target in a ground area containing iron-oxide is ferrous or non-ferrous. As a result current metal detectors are relatively limited in discrimination search depth.
It is an object of this invention to overcome some of the above difficulties or at least provide the public with a useful alternative.